ROOM TO MOVE: ACCOMMODATING WOODY DEBRIS IN STREAM CHANNELS AS FISH AND INVERTEBRATE HABITAT

Brendan Hicks (Morphum Environmental Ltd, University of Waikato), Kendall Vaughan (Morphum Environmental Ltd), Kyle Frederick (Morphum Environmental Ltd), Matthew Hall (Morphum Environmental Ltd), Jacqui McCord (Morphum Environmental Ltd), Mark Lowe (Morphum Environmental Ltd), Andrew Rossaak (Morphum Environmental Ltd)

ABSTRACT

Stream channels with trees in the riparian zone and on hill slopes frequently have in-stream woody debris as a result of natural geomorphic and erosion processes. Landslides and windthrow are common mechanisms of entry of wood into streams. Once in the stream channel, trees can become part of the channel morphology as in-stream woody debris, which can create hydrological problems. Mobile woody debris can block constrained channels and cause damage to structures such as bridges and culverts, especially in urban areas. However, in wider, unconstrained channels with flood plains that can accommodate high flows, in-stream debris provides valuable channel stability and complexity, and habitat for fish and invertebrates. Removal of woody debris can lead to channel degradation and loss of valuable habitat. Reductions in fish and invertebrate abundance result from debris removal partly from reduced pool length and depth and partly from expansion of riffle habitat, which can be expressed as changes to the pool:riffle ratio. Channel complexity, crevice habitat, and heterogeneous water velocities, on which diverse fish and invertebrate communities rely, are also reduced by removal of woody debris. Particularly badly affected are banded kōkopu, which typically inhabit pools.

Provided that a river has room to move in its flood plain during high flows without creating damage to assets, retention of in-stream debris maintains ecological function and creates stream channel habitat diversity. Functional woody debris is submerged in the low-flow channel and is often in contact with the stream bed. Debris in the channel provides a stable surface for invertebrates to live and feed, protection for fish from predation and high water velocities, and has an important role in sediment routing and storage. Woody debris suspended above the channel is not functional but is part of the debris storage reservoir and can become functional after relocation by floods. We propose considering both geomorphic and functional approaches to decisions of whether or not to remove in-stream debris.